The present application relates to technologies for manufacturing nano-scale electronic devices, and in particular, to methods of obtaining high-density aligned carbon nanotubes suitable for making such devices.
Carbon nanotubes (CNT) are a one-dimensional (1D) material having ultra-long electron mean free path. Comparing to conventional silicon-based Complementary Metal Oxide Semiconductor (CMOS) field-effect transistors, carbon nanotube field-effect transistors (CNFET) have extraordinary electronic performance and controllability over static charges, which is a hopeful to replace silicon-based chip semiconductor technologies.
CNFET based on a single carbon nanotube has low drive current, and thus is easily disturbed by external parasitic capacitance, which results in slow speed. To reduce parasitic effects and improve device performance, it is essential to increase the density of carbon nanotubes in CNFETs. The ideal density for aligned carbon nanotubes is 100-200 tubes/μm for fabricating high speed and low energy consumption carbon nanotube transistor.
Conventional CNT manufacturing technologies, however, cannot produce high-quality high-density aligned carbon nanotubes. One conventional method is the direct growth method. In this method, CNTs can be grown directly quartz substrate to obtain an average density of 10-20 CNTs/μm in a single growth cycle, and an average density of 40-50 CNTs/μm after multiple growth cycles. As the density increases, nanotubes tend to intertwine, bend, and overlap, thus blocking the subsequent growth in an aligned fashion. Thus the direct-growth method cannot produce high density and highly aligned CNTs. Another CNT manufacturing method is based on transfer using Langmuir-Blodgett (TB) membranes; it can produce large area of uniformly aligned CNTs. This method involves purification, ultrasonic treatment, and modification, which make the product CNTs very short and contain lots of defects. Thus the LB membrane method is also not suitable for manufacturing highly aligned high-density CNTs for fabricating high performance electronic devices.
U.S. Patent Publication No. 20090028779 A1 disclosed a method for making high density CNT array, including: providing substrate with a carbon nanotube array perpendicular to it; providing an elastic film; stretching the elastic film uniformly, and covering the elastic film on the carbon nanotube array; applying pressure uniformly on the elastic film; shrinking the elastic film while maintaining the pressure; and separating the carbon nanotube array from the elastic film to acquire a high-density carbon nanotube array. Nevertheless, this method can only increase the density of the carbon nanotubes along the direction perpendicular to the substrate. On the other hand the density in directions parallel to the substrate is the most important for fabricating electronic devices. In these directions, the carbon nanotubes have large contact areas with the substrate, resulting in strong Van der Waals force between the nanotubes and substrate. Shrinking the elastic film in directions parallel to the substrate tend to bend and intertwine the nanotubes. Moreover, the step of physically separating the carbon nanotubes from the elastic film may work for high density intertwined carbon nanotubes (aligned perpendicular to the substrate, i.e. CNT forest), but cannot work with lower density carbon nanotubes aligned parallel to the substrate. Furthermore, the disclosed method suffers large material loss because the carbon nanotubes are strongly attracted to and adhered to the elastic film. In sum, the disclosed method is unsuitable for making high density, high quality carbon nanotube arrays that are aligned parallel to substrate, which are essential for fabricating electronic devices.
There is therefore a critical need for making high-density carbon nanotubes that are aligned parallel to the substrates, which is essential for developing high performance carbon nanotube-based electronics.